New publication on shark bite injuries to inshore dolphins in the Kimberley

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We are pleased to announce the following publication in Marine Mammal Science:

Shark bite injuries on three inshore dolphin species in tropical northwestern Australia.

Authors: Felix Smith, Simon J. Allen, Lars Bejder and Alexander M. Brown.

Dolphins frequently bear evidence of shark bites, which can provide an indirect measure of predation pressure, but methodical assessments of bite prevalence in the scientific literature are rare. Here, we draw upon several years of photo-identification data from MUCRU research in the Kimberley region of north-western Australia to document the prevalence of shark bites on snubfin, humpback and bottlenose dolphins. With data for four discrete study sites (Figure 1), we investigated spatial and inter-specific differences in the prevalence of bites on these three sympatric species.

Figure 1. Study sites and sample sizes for Australian snubfin (Oh), Australian humpback (Ss) and Indo-Pacific bottlenose (Ta) dolphins in north-western Australia.

Key findings:

  • Shark bite prevalence varied markedly between species, with 72% of snubfin, 46% of humpback and 18% of bottlenose dolphins exhibiting evidence of shark bites.
  • The prevalence of shark bites on snubfin dolphins is among the highest recorded on any dolphins.
  • Snubfin and bottlenose dolphins at Cygnet Bay displayed a notably higher number of shark bite scars than dolphins at the other study sites (Figure 3).
  • At all sites, over 50% of shark bites on snubfin dolphins could be attributed to tiger sharks (Galeocerdo cuvier) suggesting they are a major predator of snubfin dolphins in the Kimberley region (Figure 2).

Figure 2. Shark bite wounds on bottlenose (A), snubfin (B-E) and humpback (F) dolphins. A is likely from a large carcharhinid shark (not tiger), while B-E are attributable to tiger sharks. A-E are representative of the majority of bite sizes recorded, while F illustrates a smaller bite to the dorsal fin of a juvenile humpback dolphin and is likely from a small carcharhinid shark.

Figure 3. Prevalence of shark bites on Australian snubfin, humpback and Indo-Pacific bottlenose dolphins. n = sample size for each species at each study site. Site-specific humpback dolphin data from Beagle Bay (n = 2) and bottlenose dolphin data from Roebuck Bay (n = 2) are not illustrated due to small sample sizes, but these individuals are included in the pooled data for all study sites.

Predation risk is a key factor influencing dolphin group structure and composition, habitat use, reproductive strategies and the evolution of sociality. Our findings suggest that predation risk is an important but varying influence on inshore dolphins off north-western Australia, and provide a valuable input to future investigations of the behavioral ecology of these sympatric species.

For further info and/or a copy of the full text, contact: Felix Smith, felixsmith@gmail.com

Full Citation: Smith F., Allen, S.J., Bejder, L. and Brown, A.M. (2017). Shark bite injuries on three inshore dolphin species in tropical northwestern Australia. Marine Mammal Science. doi: 10.1111/mms.12435

Abstract

Predation risk has a profound influence on the behavior of marine mammals, affecting grouping patterns and habitat use. Dolphins frequently bear evidence of shark bites, which can provide an indirect measure of predation pressure. Using photo-identification data, we investigated the prevalence of shark bites on three sympatric species of inshore dolphin, the Australian snubfin (Orcaella heinsohni), Australian humpback (Sousa sahulensis), and Indo-Pacific bottlenose dolphin (Tursiops aduncus), among four study sites in northwestern Australia. Bite prevalence varied markedly between species, with 72% of snubfin, 46% of humpback, and 18% of bottlenose dolphins exhibiting evidence of shark bites. Binomial logistic regression confirmed a high likelihood of bite presence on snubfin dolphins, and at one particular site for snubfin and bottlenose dolphins. The prevalence of tiger shark (Galeocerdo cuvier) bites on snubfin dolphins was high, and bites attributed to other carcharhinid sharks were observed on all species. While acknowledging methodological differences with other studies, the prevalence of shark bites on snubfin dolphins is among the highest reported for any dolphins, suggesting predation risk represents an important but varying influence thereon. This study provides a baseline for future investigations into the affect of predation risk on the behavioral ecology of these sympatric species.

The data presented in this study stems from research funded by the Australian Marine Mammal Centre (Project 11/23), WWF-Australia, and WAMSI (Project 1.2.4). AMB was the recipient of a Murdoch University International Postgraduate Research Scholarship. We are grateful to the Kimberley Marine Research Station, Arrow Pearls, Clipper Pearls, and Marine Produce Australia for in-kind support at study sites. This study would not have been possible without the numerous research assistants who, combined, volunteered many months of time to assist with data collection. Special thanks also go to Nyamba Buru Yawuru Country Managers (Roebuck Bay) and the Dambimangari Rangers (Cone Bay) for their enthusiastic participation in data collection.

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